Air Pollution

Pollution

Pollution may be defined as the addition of undesirable material into the environment as a result of human activities. The agents which cause environmental pollution are called pollutants.

A pollutant may be defined as a physical, chemical or biological substance released into the environment which is directly or indirectly harmful to humans and other living organisms.

Pollution may be of the following types: Air pollution, Noise pollution, Water pollution, Soil pollution, Thermal pollution and Radiation pollution.

In order to control environmental pollution, the Government of India has passed the Environment (Protection) Act, 1986 (Bhopal disaster) to protect and improve the quality of our air, water and soil.

Air pollution may be defined as the presence of any solid, liquid or gaseous substance including noise and radioactive radiation in the atmosphere in such concentration that may be directly and/or indirectly injurious to humans or other living organisms, property or interferes with the normal environmental processes.

An ever-increasing use of fossil fuels in power plants, industries, transportation, mining, construction of buildings, stone quarries had led to air pollution.

Fossil fuels contain small amounts of nitrogen and sulphur.

Burning of fossil fuels like coal (thermal power plants) and petroleum release different oxides of nitrogen and sulphur into the atmosphere.

These gases react with the water vapour present in the atmosphere to form sulphuric acid and nitric acid. The acids drop down with rain, making the rain acidic. This is called acid rain.

Acid rain corrodes the marble monuments like Taj Mahal. This phenomenon is called as Marble cancer.

Other kinds of pollutants are chlorofluorocarbons (CFCs) which are used in refrigerators, air conditioners and as pressurising agents in aerosol sprays. CFCs damage the ozone layer of the atmosphere.

The combustion of fossil fuels also increases the number of suspended particles in the air. These suspended particles could be unburnt carbon particles or substances called hydrocarbons.

Presence of high levels of all these pollutants causes visibility to be lowered, especially in cold weather when water also condenses out of the air. This is known as smog and is a visible indication of air pollution.

Filters

Baghouse filtration system is the most common one and is made of cotton or synthetic fibres (for low temperatures) or glass cloth fabrics (for higher temperature up to 2900 C).

Electrostatic precipitators (ESP)

Electrostatic precipitation can remove over 99 per cent particulate matter present in the exhaust.

The emanating dust is charged with ions, and the ionised particulate matter is collected on an oppositely charged surface.

Working

An electrostatic precipitator has electrode wires that are maintained at several thousand volts, which produce a corona that releases electrons.

These electrons attach to dust particles giving them a net negative charge. The collecting plates are grounded (relatively positive charge) and attract the charged dust particles.

The velocity of air between the plates must be low enough to allow the dust to fall.

The particles are removed from the collection surface by occasional shaking or by rapping the surface.

ESPs are used in boilers, furnaces, and many other units of thermal power plants, cement factories, steel plants, etc.

Inertial collectors

It works on the principle that inertia of SPM (suspended particulate matter) in gas is higher than its solvent and as inertia is a function of the mass of the particulate matter, this device collects heavier particles more efficiently (centrifugation is the technique).

‘Cyclone’ is a common inertial collector used in gas cleaning plants.

Scrubbers

Scrubbers are wet collectors. They remove aerosols from a stream of gas either by collecting wet particles on a surface followed by their removal or else the particles are wetted by a scrubbing liquid.

The particles get trapped as they travel from supporting gaseous medium across the interface to the liquid scrubbing medium. (this is just like mucus in trachea trapping dust)

A scrubber can remove gases like sulphur dioxide.

Catalytic converter

Catalytic converters, having expensive metals namely platinum-palladium and rhodium as the catalysts, are fitted into automobiles for reducing the emission of poisonous gases.

As the exhaust passes through the catalytic converter, unburnt hydrocarbons are converted into carbon dioxide and water, and carbon monoxide and nitric oxide are changed to carbon dioxide and nitrogen gas, respectively.

Motor vehicles equipped with catalytic converter should use unleaded petrol because the lead in the petrol inactivates the catalyst.

Apart from the use of the above mentioned devices, other control measures are:

Steps Taken to Control Vehicular Pollution

The emission standards for automobiles have been set which if followed will reduce the pollution.

Standards have been set for the durability of catalytic converters which reduce vehicular emission.

In cities like Delhi, vehicles need to obtain Pollution Under Control (PUC) certificate at regular intervals.

This ensures that levels of pollutants emitted from vehicles are not beyond the prescribed legal limits.

The price of diesel is much lower than petrol which promotes the use of diesel. To reduce the emission of sulphur dioxide, sulphur content in diesel has been reduced to 0.05%.

Earlier lead in the form of tetraethyl lead was added in the petrol to raise octane level for the smooth running of engines. Addition of lead in petrol has been banned to prevent the emission of lead particles.

Usage of alternative fuels like CNG in public transport vehicles is made mandatory in cities like Delhi. All the buses of Delhi were converted to run on CNG by the end of 2002.

CNG burns most efficiently, unlike petrol or diesel, in the automobiles and very little of it is left unburnt.

Moreover, CNG is cheaper than petrol or diesel, cannot be siphoned off by thieves and adulterated like petrol or diesel.

The main problem with switching over to CNG is the difficulty of laying down pipelines to deliver CNG through distribution points/pumps and ensuring uninterrupted supply.

Simultaneously parallel steps taken in Delhi for reducing vehicular pollution include phasing out of old vehicles, use of unleaded petrol, use of low-sulphur petrol and diesel, use of catalytic converters in vehicles, application of stringent pollution-level norms for vehicles, etc.

The Government of India through a new auto fuel policy has laid out a roadmap to cut down vehicular pollution in Indian cities.

More stringent norms for fuels means steadily reducing the sulphur and aromatics content in petrol and diesel fuels.

The goal, according to the roadmap, is to reduce sulphur to 50 ppm in petrol and diesel and bring down the level of aromatic hydrocarbons to 35 per cent.

Bharat Stage VI (BS VI) from 2020

From April 2017, BS IV norms are applicable nationwide.

October 2018: Supreme Court ordered a ban on the sale of Bharat Stage IV vehicles from April 1, 2020.

The central government had announced the April 1, 2020 deadline for adopting Bharat Stage VI emission norms by manufactures.

Bharat Stage (BS) norms

The BS norms are instituted by the government to regulate the emission of air pollutants from motor vehicles.

The norms were introduced in 2000.

The norms limit the release of air pollutants such as nitrogen oxides, carbon monoxide, hydrocarbons, particulate matter (PM) and sulphur oxides from vehicles using internal combustion engines.

The norms are meant to be adopted by using appropriate fuel and technology.

As the stage goes up, the control of emissions become stricter.

BS IV and BS VI norms are based on similar norms in Europe called Euro 4 and Euro 6.

As decided initially, BS V would have been rolled out by 2021 and BS VI in 2024, but leapfrog to BS VI norms by 2020 (skipping BS V) had to be done because of the carbon footprint obligations.